Capital Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Equity [Abstract]
Percent of entity owned by current and retired employees			100.00%
Percent of entity held in voting trust	83.00%		83.00%
Common, stated value (in dollars per share)	$ 20		$ 20		$ 20
Dividends, Common Stock, Cash	$ 5,850	$ 5,286	$ 17,587	$ 15,910
Preferred Stock, Shares Authorized	10,000,000		10,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01
Preferred Stock, Shares Outstanding	0		0		0